1. Field of the Invention
This invention relates to a method for separating and recovering desired plastic particles from a mixture containing two or more plastic particles. More particularly, this invention relates to a process for the separation of material made of plastics having similar specific gravities. In particular, the process relates to separating a mixture of polyvinyl chloride and another plastic having a similar specific gravity to that of polyvinyl chloride into its component particles.
2. Description of the Art
Recycling and reclaiming waste materials has become important in abating pollution, preserving the environment, and conserving energy due to the reprocessing of the reclaimed waste materials. Because of the wide variety of articles manufactured today using various plastics, discarded plastics have become a large source of waste materials.
Effectively separating and recovering various plastics is important in recycling waste plastic materials. Waste plastic materials are diverse and may be composed of numerous different types of materials such as polyvinyl chloride, polyethylene, polypropylene, and polyethylene terephthalate, for example. These materials are found in municipal plastic scrap which includes bottles and other packaging materials, electrical wire and cable scrap, and industrial plastic scrap.
Numerous methods have been developed for separating plastic materials. Generally, a plastic material can be separated into its components by various floatation processes. U.S. Pat. No. 3,925,200 generally discloses such a process. U.S. Pat. No. 4,578,184 discloses the use of density adjustors for the separation medium used in flotation methods. This patent discloses the use of plasticizers to adjust the density of the solution used to separate polyethylene and polyvinyl chloride. These well known flotation methods are quite effective for mixtures of plastic materials having different specific gravities, such as polyethylene terephthalate and polyethylene or polyvinyl chloride and polyethylene. However, these known methods are ineffective for mixtures of plastics having similar specific gravities such as for example, polyvinyl chloride (hereinafter "PVC") and polyethylene terephthalate (hereinafter "PET").
The effective, safe, efficient, easy and economical separation of mixtures of plastics with similar specific gravities is important because of the abundance of articles manufactured from multicomponent plastics such as plastic bottles, electrical cable and wire insulation and packaging materials. Often, when various different types of plastics are commingled, there may be problems with reprocessing the materials and with the properties of the resultant products. For example, when PVC particles are commingled with PET particles, the plastic does not reprocess as well as either PVC or PET by itself. PET contaminated by PVC cannot be reprocessed because PVC degrades at lower temperatures than the PET processing temperature. Thus, no useful product can be obtained. On the other hand, PVC contaminated by PET is not useful because PET does not melt at the PVC processing temperatures. Rather, the PET contaminant particles are present as defects that significantly lower the physical properties of PVC. Therefore, a need exists to effectively separate mixtures of plastics containing plastic particles of similar specific gravities. In particular, a need exists to effectively separate a mixture containing PVC and another plastic having a similar specific gravity to that of PVC.
In order to separate mixtures of plastics, several techniques have been developed. A thermal process for separating PVC and PET is described in "PVC Melt Point Keys Scrap Separation," Modern Plastics, June 1990, pp. 15-16. A mixture of PVC and PET is metered onto a conveyor belt which transports the mixture through a heating chamber. The heating chamber is maintained at the melt point for PVC. As the PVC melts, it adheres to the conveyor belt. At the end of the conveyor belt, the PVC is scraped off the belt and the PET drops off the conveyor belt. The two components are thereby separated.
Zielinski in U.S. Pat. No. 5,061,735, describes another thermal process for the separation of mixtures of plastics. Zielinski heats the mixture to a temperature sufficient to cause one of the plastic materials to adhere to itself while simultaneously agitating the mixture so that adhering particles agglomerate to form larger particles. The contaminant material particles, which could be formed of another plastic, do not adhere to each other or to the agglomerated thermoplastic particles. The mixture is cooled and agitated. The larger agglomerated particles are then separated from the smaller contaminant particles using a series of screens.
Both of these methods necessitate heating the particles close to the melting point of one of the plastics. This heating may degrade the properties of at least one of the components of the plastic mixture such as PVC, rendering the recycled component unusable.
Electromagnetic identification is another means to separate mixtures of plastic. For example, chlorine scanning is used to separate PET from PVC when the two are mixed together. In this technique, PVC is segregated from the PET by electromagnetic identification of the chlorine atoms. ("PVC Melt Point Keys Scrap Separation", Modern Plastics, June 1990, pp.15-16) This technique is expensive due to the equipment needed to carry out the segregation. In addition, mechanical problems associated with the process render contaminated plastic particles which are not readily reprocessed and possess inferior physical properties and appearance. This is especially true in applications where very thin wall parts are required.
Another technique useful in separating mixtures of plastic particles is a solvent processing technique. This technique is dangerous because the solvents used in the technique can ignite due to their low flash points. In addition, the solvents release harmful vapors. An example of a solvent processing technique is disclosed in U.S. Pat. No. 4,617,111 to Grimm et. al. for the separation of PVC from PET particles. Grimm et. al. add a solution composed of a solvent of polyvinyl chloride having a density of less than 0.95 grams per cubic centimeter and water to flakes of PET and PVC. The flakes are conditioned in the solution for a predetermined time so that the PVC flakes absorb the solvent whereas the PET flakes do not because they are insoluble in the solvent. The PVC flakes are then separated from the PET flakes and recovered. The solvent must then be extracted by an expensive process from the PVC flakes before the PVC can be reprocessed.
Although these known methods are useful, there still exists a need for easy, safe, efficient and inexpensive separation of mixtures of plastic particles where the particles have similar specific gravities.
Accordingly, a primary objective of the invention is to effectively and efficiently separate and reclaim the plastic particles having similar specific gravities from mixtures containing the plastic particles which cannot be separated by conventional flotation or centrifugation techniques.
An additional object of the invention is to economically reclaim each of the component plastic particles from mixtures containing the plastic particles having similar specific gravities without additional recovery steps other than normal rinsing and drying.
Another object of the invention is to easily and economically separate and reclaim plastic particles having similar specific gravities from mixtures of the particles.
Yet a further object of the invention is to recover plastic particles having similar specific gravities from mixtures containing the particles so that they can be effectively reprocessed or reused.
Still another object of the invention is to safely and efficiently recover plastic particles having similar specific gravities from mixtures containing the particles.
Yet an additional object of the invention is to recover PVC from a mixture containing PVC and another plastic having a similar specific gravity to that of PVC.